Means for and method oe reducing singing in repeaters



Patented June 3, 1924.

UNITED STATES PATENT OFFICE.

LLOYD ESPENSCHIED, OF QUEENS, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A- CORPORATION OF NEW YORK.

MEANS FOR AND METHOD OF REDUCING SINGING IN REPEATERS.

Application filed June 17, 1921.

To all whom it may concern Be it known that I, Lnovn Esrnivsouinn, residing at Queens, in the county of Queens and State of New York, have invented certain Improvements in Means for and Methods of Reducing Singing in Repeaters, of which the following is a specification.

This invention relates to transmission systems, and more particularly to radio or wire transmission systems in which repeaters are employed.

In accordance with the present invention, it is proposed to operate a transmission system in such a manner that the various fre quencies transmitted in a signal band will be subjected to substantially the same at tenuation, and the tendency to sing at repeater points will be reduced or substan tially eliminated.

As is well known, when signaling currents involving a band of frequencies, as for example, telegraph or telephone currents, are transmitted over a long conducting system, two considerable diiiiculties are involved. 5 All of the frequencies are subjected to very large attenuation, making it necessary to amplify the frequencies at various intervals.

Furthermore, much distortion is encountered due to the fact that the attenuation is not equal for all frequencies, but is generally much greater for the higher frequencies than for the lower frequencies. In the case of long conducting systems, this distortion due to variation of attenuation with frequency, becomes so great as to lower the intelligibility of telephone transmission by a considerable amount. The necessity for amplifying the various frequencies at certain intervals necessitates the use of repeaters. and where two-way transn'iission is involved a tendency to sing or howl is inherent in repeater systems heretofore known in the art. This singing tendency is due to the fact that the energy transmitted over one channel of the repeater tends to be in part transmitted back in the opposite direction over the other channel. due to unbalance, and if the energy fed back is of the same frequency, and is sufficient in amount to apply to the original channel a new force equal to or greater than the original force, the repeater circuit will oscillate continuously.

In accordance with the present invention, it is proposed to overcome the difficulty due to distortion by inverting the signaling band Serial No. 478,378.

at the repeater points so that the band will be transmitted in normal condition over a line section connecting two repeater points, and in inverted frequency relation over the next line section connected in tandem therewith, etc. By making the total number of sections in which the band is transmitted in inverted frequency relation equal in length to the total number over which the band is transmitted in normal condition, various frequencies in the signaling band will be subjected to substantially the same attenuation so that distortion will be to a large extent clilniinited.

The ditiiculty encountered in connection with the general tendency of the circuit to attenuate transmission is overcome as is customary by the provision of amplifying arraugements at repeater points, and the tendency of the amplifying arrangements to sing .is overcome by so arranging matters that the bands transmitted in opposite directions over the line section associated with one end of each repeater will not quite coincide with each other, but will be slightly displaced in the frequency spectrum with regard to each other. although they overlap over the greater part of the frequency arrangements. As the repeaters in themselves are arranged to produce a frequency translation necessary to invert the transmission bands, as well as to produce displacement above referred to; the repetition of energy about the closed circuit formed by the repeater channels involves a change of frequency so that the tendency to continuously sing at a given frequency is eliminated.

Further details of the invention may be understood from the following description thereof, when read in connection with the accompanying drawing, Figure 1 of which is a simplified diagram of one form of apparatus for practising the invention, Fig. 2 of which is a system of curves illustrating the frequency inversion and frequency shifting taking place at the repeater points, while Fig. 3 is a schematic representation of apparatus for exen'iplifying the invention in connection with the carrier system.

Referring to Fig. l. a telephone transmission system is illustrated having two repeater points and preferably the repeater points are so located that the length of line inter-connecting the two repeater points will. be equal to the total lengtl'is of the two terminal line sections, so that by transn'iitting the band in normal condition over the terminal sections, and in inverted condition over the intermediate section, the equalization of attenuation above referred to may be accomplished. It is, of course, obvious that. additional repeater points may be provided, the only requirement being that the fre quency band be transmitted over substantially the same total length of circuit in inverted condition as that over which it is transmitted in normal condition. At station A the. line sections L and L are interconnected by a circuit of the 22 repeater type. comprising channel RVV for transmitting from east to west, and channel PE for transmitting from west to east. The channel Riv, includes an amplifying apparatus M which may he of any well known clnu-acter. such as, for example. a vacuum tube amplifier and a modulator. The modulator is shown as being supplied with a carrier frequency, and in the output circuit of the modulator a band filter Bl is provided for selecting the lower side band due to modulation. Assuming that the voice range to be transmitted extends between the frequencies 200 cycles and 2,000 cycles, the band filter BF may be arranged to have cut-off points at 200 cycles and 2,000 cycles respectively.

The corresponding channel RE for transmitting in the opposite direction, is provided with a similar modulating and amplifying apparatus M in the output circuit of which is a band filter BF, haying charac teristics similar to the band filter BF In the case illustrated the bands transmitted in opposite directions over the line section L are to be slightly offsetand cumulative frequency shifting to take place about the closed path formed by the two repeater channels. For this purpose. the carrier frequencies supplied to the modulators M and M, are different, the frequency supplied to the modulator M being illustrated as 2300 cycles, and that to the modulator M, as 2200 cycles.

The apparatus associated with the repeater circuit at station B is similar to that already described, except that the band tilt: 1" BF in the output circuit of the modulator M will have its cut-off points slightly different from those of the band filter BF at station A. In the assumed case, the cut off points of the band filter BF are 300 cycles and 2100 cycles respectively. The apparatus as illustrated is arranged so that the hands transmitted in opposite dire 'tions over the line sections L, and L will occupy the same frequency range and will be transmitted in normal frequency order. The band transmitted from west to east over the line section L, will occupy the same fre quency range as the hands transmitted over line sections L and L but will:be in inverted order, and the band transmitted in the opposite direction, While in the inverted order, will b slightly shifted in frequency.

Further details of the invention may be now understood from a description of the operation of Fig. 1, which is as follows: Considering the case of a 200 cycle frequency component of the voice current, transmitted from west to east, this frequency incoming from the line section L passes into the repeater channel Rlfl and modulates with a frequency of 2200 cycles in the modulating amplifier M thereby producing side frequencies of 2000 cycles and 24:00 cycles. The upper frequency is eliminated by the band filter BF] but the 2000 cycle frequency is selected by said band filter and transmitted over the line section L At station B this frequency enters the channel RE, and by modulation with the 2200 cycle frequency supplied to the modulators M two side frequencies of 200 cycles and -i200 cycles are produced. The upper one is eliminated by the band filter BF and the 200 cycle frequency is selected and transmitted over the line section L A 200 cycle frequency transmitted in the opposite direction passes into the channel law and is modulated with the 2300 cycle frequency supplied by the modulator M thereby producing frequencies of 2100 cycles and 2500 cycles, the former of which is selected by the baud filter BF and transmitted over the line section L This frequency then passes into the channel RdV, and is modulated with a frequency of 2800 cycles to produce side frequencies of 200 cycles and 1400 cycles. The 200 cycle is selected by the band filter BF and transmitted over the line section L It will be observed that for transmission over the line sections L and L the frequency relations are normal in both directions, and the bands will occupy the same range. For transmission over the line section L. however, the frequency relations are inverted and. owing to the displacement in frequency as regards transmission. in opposite directions, the frequency relations are not the same for the oppositely transmitted hands, a difference of 100 cycles being involved in the assumed case. This slight offsetting of the inverted frequencies in the section L will produce no serious effect on the tendency of the system to equalize the attenuation and will have very beneficial results with respect to the prevention of Singing in the repeater. The operation of the circuit in preventing singing will be clear, considering what takes place in the repeater at station A: for example. a frequency of 200 cycles incoming from a line L is converted by the modulator M and band filter B F into a frequency of 2000 iii cycles, as already described. Some of the energy thus converted is transmitted. back into the channel RW due to unbalance, and the 2000 cycle frequency passing into this channel by modulating with the frequency of 2300 cycles supplied to the modulator M produces frequencies of 300 cycles and 4300 cywles respectively. The band filter BF, eliminates the upper of these frequencies, and part of the 300 cycle frequency passes back again into the channel Itlil, due to run balance. The frequency is now converted to i900 cycles and passing through the channel KW, is converted to e00 cycles, etc. This frequency conversion by steps of 100 cycles, it continued, results in a conversion of the fed back energy into an ultimate frequency which lies without the cut-oil limits of the filters, so that the energy tending to sing is always finally suppressed, and during the feeding bacl: operation it is changed in frequency so that singing does not actually occur.

The inversion of the frequency band and theol'lf-setting of the inverted bands transmitted east and west may be more clearly understood from Fig. 2 of the drawing. In this figure the band designated 0 illustrates the condition obtaining in the line section L,, the lower frequencies of the band being represented by heavier arrows than the up per frequencies. Similarly the band designated C indicates the conditions obtaining in the line section L,,. In both cases it will be observed that the bands occupy the same l'remiency range for transniissiiini opposite directions, this being represented by the double pointed arrows. The band representing east bound transmission is inverted at repeater point i-i. by modulation as described and appears in the intermediate line section L as hand D, occupying an inverted. range from 200 to 2000 cycles. At the next repeater point B, this band is again reversed and comes out in normal condition, as represented by the band C. For east to west transmission, the band (7 is translated at the repeater point 13 to an inverted band I), which because of the different carrier frequency employed for the west-bound channel, occupies afrequency band which offset with respect to that of the eastbound band. The west-bound band B again translated at repeater point A. and arrives right side up and in the original fre quency range at the terminal connected to the line section L lrom the above it will be appreciated that the n (in-singing repeater featin'e of the pres out invention is intimately involved with the feature of equalizing the atteni'lation, in that it concerns by virtue of a shift in fro rpiency at the repeater point. the operation. of alternate sections of the line at different 't'rrupiency relations. Considering the scheme bands of the east and west channels in order that a net frequency conversion obtains in the singing path of the repeater circuit. Furthermore, this frequency shift cannot well be etl ected by modulation with a low frequency carrier such, for example, as .100 cycles, without seriously inj uring the quality of transmission. This follows from the fact that two overlapping bands would be set up under these condi ions, one representing the sum term and the other the difference term of modulation. It should also be noted that it is sufficient, for the purpose of preventing sustained repeater singing, to shift in the repeater the frequencies of but one of the oppositely directed channels. the eastbound channel is the one shifted in frequency, for example, then the westbound channel may be transmitted in the usual way without change of frequency in the repeaters. This would not, however, result in attenuation equalization in both directions.

lVhile the present arrangement is essentially a carrier current type of system, it should be observed that the avoidance of sustained. singing of the repeater is obtained without displacing the east and west channels into entirely diiferent frequency ranges, but that these channels are merely offset in fre piency. and except for the offsetting, occupy substantially the same range as is eniployed for or-rlinary telephone transmission.

The general scheme is, of course, also applicable to a complete iarrier cliirrcnt trans mission system, as illustrated in F' Here the east and west channels, although overlapped in their frequency ranges, are offset for the purpose of preventing singing: in the repeater. In Fig. 3 it is assumed that the transmission over line sections l4, and L is at audible frequencies. and the transmission over the line section L, involves carrier transmission. The carrier employed for transmission from west to cast being 10,000 cycles, and from east to west being 10,100 cycles. The operation of the circuit is so nearly identical with Fig. 1, except for the difference in the frequencies involved that no further description is deemed necessary. Fintherniore the carrier currents may obviously be transmitted by radio instead of by wire. in which case the radio station comprises a repeating station repeating between the wire and the other transmission paths.

It will, of course, be understood. that the frequencies referred to in this description and illustrated in the drawing, as well a. the types of apparatus disclosed, are for &

illustrative purposes only and the invention is not to be understood as limited thereto.

, It will also be obvious that the general principles herein disclosed may be embodied in many other organizations widely differ ent from those illustrated, without departing from the spirit of the invention as defined in the following claims.

What is claimed is:

1. The method of preventing singing in a two-way repeater circuit, which consists in shifting the frequency for transmission in one direction through the repeater, so that a frequency translation occurs for each excursion around the singing path of the repcater.

The method of preventing singing in a two-way repeater circuit, which consists in producing a shift in frequency for transmission in each direction through the repeater, whereby the energy tending to cause singing is translated in frequency for each excursion around the singing path of the repeater.

3. The method of preventing singing in a two-way repeater circuit, which consists in inverting the signaling band for transmission through the repeater in one direction, and shifting the frequency for transmission in one direction so that a frequency translation of the energy tending to produce singing occurs for each excursion around the singing path of the repeater.

et. The method of preventingsinging in a two-way repeater circuit, which consists in modulating the transmitted band for transmission in one direction through the repeater with a frequency so chosen that a shift in frequency occurs whereby a frc quency translation of the energy tending to produce singing occurs for each excursion around the singing path of the repeater.

5. The method of equalizing attenuation over a transmission circuit and preventing singing at repeater points, which consists in inverting the transmitted bands at each repeater point, and in shifting the frequency for transmission in at least one direction through the repeater point, so that oppositely directed bands will only partially overlap.

6. The method of equalizing the attenuation in a transmission circuit and preventing singing at repeater points, which consists in inverting the transmitted signaling bands at each repeater point by niodulating the band with the carrier frequency, and shifting the frequency of one inverted band with respect to the inverted band transmitted in the opposite direction.

7. The method of equalizing attenuation over a transmission circuit. and preventing singing at repeater points, which consists in modulating the bands transmitted in opposite directions at the repeater points with carrier frequencies so chosen that the side bands resulting from modulation in opposite direction will be slightly offset through each other.

8. The method of equalizing attenuation over a transmission circuit, and preventing singing at repeater points, which consists in modulating the bands transmitted in 0pposite directions at the repeater points with *arrier frequencies so chosen that the side bands resulting from modulation in opposite directions, will be slightly offset with regard to each other, and selecting one side band resulting from modulation to the exclusion of the other side band.

5), in a signaling system, a transn'iission circuit, a plurality of repeater points therein, means at each repeater point for inverting a transmitted signaling band, and means for offsetting the bands transmitted in opposite directions.

10. In a signaling system, a transmission circuit, a plurality of repeater points therein, means at each repeater point for modulating the signaling band transn'iitted in one direction to produce an inversion of the band, and means to shift the band transmitted in the opposite direction.

ll. In a signaling system, a transmission circuit, a plurality of repeater points therein, means at each repeater point for modulating the band transn'iitted in each direction through the repeater to produce an inversion of the band, the frequencies used for modulation in opposite directions being so related to ach other in the frequency spectrum as to produce an offsetting of the bands.

12. In a signaling system, a transmission circuit, a plurality of repeater points therein, means at each repeater point for modulating the bands transmitted in opposite directions with carrier frequencies so related to each other in the frequency spectrum as to produce side bands slightly offset for transmission in the opposite directions, and means for selecting a side band in each case, at least one of the side bands selected being inverted with respect to the modulating band.

13. In a signaling system, a repeater comprising means for repeating oppositely directed signaling bands, means associated with said repeater for inverting one of said signaling bands, and means for offsetting the bands transmitted in opposite directions.

14. In a signaling system, a repeater comprising means for repeating oppositely directed signaling bands, means associated with said repeater for modulating the signaling band transmitted in one direction to produce an inversion of the band, and means to shift the band transmitted in the opposite direction.

15. In a signaling system, a repeater comprising means for repeating oppositely directed signaling bands, means associated With said repeater for modulating the band transmitted in each direction through the repeater to produce an inversion of the band, the frequencies used for modulation in opposite directions being so related to each other in the frequency spectrum as to produce an ofii'setting of the bands.

16. In a signaling system, a repeater comprising means tor repeating oppositely directed signaling bands means associated with said repeater for modulating the bands transmitted in opposite directions With carrier frequencies so related to each other in the frequency spectrum as to produce side bands slightly oiiset for transmission in opposite directions, and means for selecting a side band in each case, at least one oi the side bands selected being inverted with re spect to the modulating band.

.ln testin'iony whereof, I have signed my nan'ie to this spe itication this 15th day of June, 1921.

LLOYD ESPENSClIl'ED. 

